reintroduce /dev/urandom arc4random for Haiku

Their arc4random implementation hasn't made it to a stable release.

docs/CHANGELOG.md

@@ -20,6 +20,4 @@ Changes in libobsd master:
 - Add C++ support
 - Add timespeccmp(), timespecsub(), and other timeval/timespec macros from
   sys/time.h
-- Delete old /dev/urandom backend for arc4random now that HaikuOS doesn't need
-  it anymore
 

src/stdlib/arc4random_buf_dev_urandom.c

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2022 Guilherme Janczak <[email protected]>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+/* Haiku doesn't have a proper programming API for generating random numbers, so
+ * it's impossible to provide the semantics of arc4random_buf(). We'll
+ * approximate them with /dev/urandom.
+ */
+void
+arc4random_buf(void *_buf, size_t nbytes)
+{
+	int errnum;
+	int fd;
+	unsigned char *buf = _buf;
+	ssize_t nr;
+	size_t unr;
+	errnum = errno;
+
+	while ((fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC)) == -1) {
+		if (errno == EINTR)
+			continue;
+		abort();
+	}
+
+	while (nbytes > 0) {
+		nr = read(fd, buf, nbytes);
+		if (nr == -1) {
+			if (errno == EINTR)
+				continue;
+			abort();
+		} else if (nr == 0) {
+			abort(); /* /dev/urandom should never EOF. */
+		}
+
+		unr = nr;
+		buf += unr;
+		nbytes -= unr;
+	}
+
+	close(fd);
+	errno = errnum;
+}

src/stdlib/meson.build

@@ -47,6 +47,8 @@ if stdlib['arc4random_buf'][2]
                                            dependencies: bcrypt_dep)
     deps += bcrypt_dep
     impl = 'arc4random_buf_BCryptGenRandom.c'
+  elif host_machine.system() == 'haiku'
+    impl = 'arc4random_buf_dev_urandom.c'
   elif host_machine.system() == 'emscripten'
     impl = 'arc4random_buf_emscripten.c'
   elif cc.has_function('getrandom', args: args,